Changes in human plasma melatonin profiles in response to 50 Hz magnetic field exposure

: The effects of power‐frequency magnetic fields on nighttime plasma melatonin were studied in a group of 30 adult male human subjects. Exposure consisted of 20 μT (200 mG) at 50 Hz (circularly polarized) at certain times in relation to the predicted time of onset of rise in melatonin concentration for a particular individual (the time of onset was predicted from a previous screening night). Response to this exposure was compared to sham‐exposure (in random order). When exposure preceded onset of rise, a significant delay in onset time relative to sham‐exposure of approximately half an hour was observed, with indications (marginally significant) of a reduction in maximum melatonin level. Analysis of distribution of time‐delays is consistent with two populations: those individuals who respond (around 20%) and those that do not. Magnetic fields generated by square‐wave currents produce more marked reductions in the maximum level when compared to sinusoidal waveforms, but there was no significant difference in onset time.